The function of an audio amplifier is to take an input audio signal, amplify and process it as necessary, and produce an output audio signal. Radiated EM (electro-magnetic) signals, such as those from nearby wireless equipment, having a transmitted power envelope with frequency components in the audio band, may be picked up at some point in the audio equipment. This interference can be inadvertently demodulated into audioband components in the audio amplifier circuitry (for example by a FET in an electret microphone) and added to the desired signal. These interference signals may then be output along with the desired signal resulting in an undesired noise component in the output signal.
Acoustic amplifiers include audio amplifier circuitry and thus are susceptible to the above-described problem of EM interference. In an acoustic amplifier, an input acoustic signal is converted to an audio signal which is input to the audio amplifier circuitry where it is amplified and processed. The output of the audio amplifier circuitry is reconverted into an amplified output acoustic signal.
EM interference can be a serious problem in hearing aids in which amplifiers with a large gain and amplitude compression are usually employed. The most important input to a hearing aid is a desired acoustic input, and the most important output of a hearing aid is a processed and amplified acoustic output. The desired acoustic signal is transduced into an electrical signal, processed and amplified by electronic components in the hearing aid, and converted back or transduced into the output acoustic signal. Depending on the frequency characteristics and power envelope of any interfering EM signals, these can be transduced along with the desired electrical signal to produce an audible interference component in the amplified sound produced by the hearing aid.
Typically, for an EM source to cause interference in a hearing aid, the source must be quite close to the hearing aid, and must possess certain EM characteristics such as a non-constant envelope. For example EM radiation from television sets, computer monitors, and neon lighting systems can interfere with hearing aid operation. More recently, digital cellular telephony, whose signals meet these conditions has become a problem in this area. With the increasingly widespread use of digital cellular telephones, a technique for eliminating their interference effects upon hearing aids is desired.
It is common in many types of audio equipment to employ techniques for reducing or cancelling noise or interference. In contrast to the above described situation in which an inadvertently received EM signal interferes with an internally generated audio signal, existing systems deal with interfering signals which are received in the same physical manner as the desired signals. For example, in hearing aids which have a microphone and a speaker portion, acoustic feedback from the speaker into the microphone may exist, and adaptive equalization may be employed in the hearing aid to reduce or minimize the negative effects of the feedback upon the operation of the hearing aid.
Three existing systems which employ such a technique for reducing acoustic feedback are disclosed in U.S. Pat. No. 5,412,735 by Engebretson et al. which issued May 2, 1995 entitled "Electric Filter Hearing Aids and Methods", U.S. Pat. No. 5,475,759 by Engebretson et al. which issued Dec. 12, 1995 entitled "Adaptive Noise Reduction Circuit for a Sound Reproduction System" and U.S. Pat. No. 5,402,496 by Soli et al. which issued Mar. 28, 1995 entitled "Auditory Prosthesis Noise Suppression Apparatus and Feedback Suppression Apparatus Having Focused Adaptive Filtering".
As another example, noise cancellation systems exist for the purpose of cancelling acoustic background noise. These systems employ a main microphone near the desired sound source, and a noise reference microphone near the source of the noise, for example, a vent fan. The main microphone will s till pick up unwanted noise from the fan. The inputs from the main microphone and the noise microphone are combined so as to remove from the main microphone the effects of the ventilation noise. The performance of such active noise cancellation systems is also compromised when the noise reference microphone can pick up some of the desired sound signal as well as the acoustic noise signal.